cache Class avoid PermGen space OOM

[xiaoheng1]

What is the purpose of the change

cache Class avoid PermGen space OOM

see more detail from #6742

Brief changelog

XXXXX

Verifying this change

XXXXX

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[codecov-io]

Codecov Report

Merging #7374 (6ecb758) into master (d009d0c) will decrease coverage by 0.03%.
The diff coverage is 64.00%.

@@             Coverage Diff              @@
##             master    #7374      +/-   ##
============================================
- Coverage     58.72%   58.69%   -0.04%     
+ Complexity      425      423       -2     
============================================
  Files          1043     1043              
  Lines         42471    42475       +4     
  Branches       6212     6215       +3     
============================================
- Hits          24942    24930      -12     
- Misses        14748    14761      +13     
- Partials       2781     2784       +3     

Impacted Files	Coverage Δ	Complexity Δ
...n/java/org/apache/dubbo/common/bytecode/Proxy.java	72.22% <64.00%> (ø)	0.00 <0.00> (ø)
.../apache/dubbo/rpc/protocol/AsyncToSyncInvoker.java	62.06% <0.00%> (-10.35%)	0.00% <0.00%> (ø%)
...dubbo/remoting/exchange/support/DefaultFuture.java	84.61% <0.00%> (-3.42%)	0.00% <0.00%> (ø%)
...c/main/java/org/apache/dubbo/rpc/RpcException.java	80.00% <0.00%> (-3.34%)	0.00% <0.00%> (ø%)
.../dubbo/remoting/transport/netty4/NettyChannel.java	64.35% <0.00%> (-2.98%)	0.00% <0.00%> (ø%)
...e/dubbo/registry/integration/RegistryProtocol.java	58.49% <0.00%> (-2.84%)	0.00% <0.00%> (ø%)
...he/dubbo/common/threadpool/ThreadlessExecutor.java	94.23% <0.00%> (-1.93%)	0.00% <0.00%> (ø%)
...a/org/apache/dubbo/monitor/dubbo/DubboMonitor.java	86.66% <0.00%> (-1.91%)	16.00% <0.00%> (ø%)
...e/dubbo/remoting/transport/netty4/NettyClient.java	69.56% <0.00%> (-1.45%)	0.00% <0.00%> (ø%)
...ubbo/metadata/store/redis/RedisMetadataReport.java	23.37% <0.00%> (-0.99%)	5.00% <0.00%> (-1.00%)
... and 17 more

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[AlbumenJ]

do not merge before check if usable